Sanity check SigmetForecastAt visitor and reconstructor

[kasarit]

PartialOrCompleteTimeInstant t = sigmet.getForecastGeometries().get().get(0).getTime().get();
    tim=String.format(" AT %02d%02dZ", t.getHour().getAsInt(), t.getMinute().getAsInt());
}
return Optional.of(this.createLexeme("FCST"+tim, LexemeIdentity.OBS_OR_FORECAST));

The lexeme identity is different from how the lexing visitor identifies the lexeme.